1. Field of the Invention
The present invention relates to an optical modulation element module, more particularly, to an optical modulation element module capable of being suitably used in an analog optical transmission system.
2. Description of the Related Art
In an analog optical transmission system, when an optical CATV transmission system is placed in 1.55 μm band, a problem occurs in the light dispersion in a fiber. Therefore, a modulation method which uses an external modulator for acquiring preferable transmission properties has been used. In modulators used for the above method, it is important to prevent wavelength chirping which is generated by intensity modulation. For this reason, Mach-Zehnder type external optical modulator (hereinafter, referred to as MZ type optical modulation element) using LiNbO3 (hereinafter, referred to as LN), in which the wavelength chirping is hardly generated, is widely used.
The MZ optical modulation element includes a Mach-Zehnder type waveguide unit (hereinafter, referred to as MZ waveguide unit) which is configured by a branching unit, such as a Y-branch or a coupler, for branching inputted light, two branched waveguides for propagating each branched light component, and an interfering unit, such as the Y-branch or the coupler, for multiplexing two light components. A modulation electrode is disposed along the branched waveguides. The modulation electrode modulates phases of the light components which propagate inside the branched waveguides on the basis of the modulation signal from a modulation signal input unit such as a connector. A region, that the light component inside the branched waveguide is phase-modulated by an electric field formed by the modulation electrode, is referred to as an interaction portion.
On the basis that the phase-changes in the respective branched waveguides are φ1 and φ2, when φ1=−φ2, a zero chirp modulator that the chirping is not generated in principle by modulating the intensity may be obtained. The zero chirp modulator may be, for example, a modulator having symmetric structure using an X-cut LN substrate as shown in FIG. 1, a modulator (hereinafter, referred to as a dual modulator) having two modulation electrodes using a Z-cut LN substrate as shown in FIG. 2, and a modulator (see Japanese Unexamined Patent Application Publication No. 2003-202530) which domain-inverts a part of the substrate by using a Z-cut LN substrate as shown in FIG. 3, and so on.
In the analog transmission modulator, an analog transmission modulator is not configured by a single MZ waveguide, but the phase-modulator for suppressing fiber Simulated Brillouin Scattering (SBS) is integrated and a plurality of optical modulation elements are connected to each other in order to linearize a transfer function of the MZ type optical modulator. Accordingly, it is important to reduce the size of each element and control the increase in the driving voltage as much as possible.
An overview of the LN modulator is shown in FIGS. 1A and 1B. An optical modulation element 3 has an MZ type waveguide which includes an input waveguide 5, a Y type branched waveguide for branching a light component, branched waveguides 6, 7, a Y type branched waveguide for multiplexing light components, and an output waveguide 8 on an X-cut LN substrate 4. Additionally, on the LN substrate 4 including the waveguide, a modulation electrode 12, ground electrodes 11, 13, and a bias electrode 14 are formed.
FIG. 1B is a cross-sectional diagram of the optical modulation element 3 taken along one dot chain line A in FIG. 1A.
One end of the modulation electrode 12 of the optical modulation element 3 is electrically connected to a modulation signal input unit 15 and the other end is connected to an RF terminal 16. Also, the bias electrode 14 is electrically connected to a bias controlling DC input unit 17. The optical modulation element 3 is hermetically sealed in a metal case 18.
The light is introduced into the optical modulation element 3 from an optical input means 1, which is provided outside the metal case 18, through the optical fiber 2. And then, the light outputted from the optical modulation element 3 is derived to an optical output means 10 through an optical fiber 9. A modulation signal for driving the optical modulation element 3 or a DC signal for controlling the bias are inputted from the external of the metal case 18 through the modulation signal input unit 15 or the DC input unit 17 for controlling the bias.
The X-cut LN modulator shown in FIG. 1 generally has low modulation efficiency, thus, it has a problem in that the driving voltage becomes higher. Especially, when using the optical modulators in multiple-stage, the length of each optical modulation element is restricted. Therefore, the driving voltage becomes higher and an amplifier for driving the modulator is needed to have high power. When integrating phase modulators, it is difficult to reduce the driving voltage due to the structure of the X-cut LN modulator. There is a problem in that the phase modulation unit occupies most of the interaction length and the interaction length of the intensity modulation unit is restricted. In X-plate LN crystal, a frequency response of modulation properties is deteriorated by acousto-optic (AO) effect. Accordingly, a measure for preventing the above-mentioned deterioration is required.
Next, an overview of a dual modulator will be described with respect to FIGS. 2A and 2B. Hereinafter, the like parts as the above-described parts will be denoted by the same reference numerals, and the description thereof will be omitted. In an optical modulation element 20, MZ type waveguides 5 to 8 are formed on a Z-cut LN substrate 21. Additionally, modulation electrodes 23, 25, ground electrodes 22, 24, 26, and a bias electrode 31 are formed on the LN substrate 21 including the waveguides.
FIG. 2B is a cross-sectional diagram of the optical modulation element 20 taken along one dot chain line A in FIG. 2A.
In the modulation electrodes 23, 25 of the optical modulation element 20, one end of each modulation electrode is electrically connected to the modulation signal input units 27, 28, and the other end is connected to RF terminals 29, 30, respectively. Additionally, a bias electrode 31 is electrically connected to the bias controlling DC input unit 17. The optical modulation element 20 is hermetically sealed in a metal case 18.
A modulation signal and a bias controlling DC signal for driving the optical modulation element 20 are inputted from the outside of the metal case 18 through the modulation signal input units 27, 28 and the bias controlling DC input unit 17.
As shown in FIG. 2, the dual modulator using the Z-cut LN substrate may have a low driving voltage and may reduce property-deterioration according to the AO effect. However, it is needed to input two complementary modulation signals as input signals. For this reason, two-output type driver is needed in a peripheral circuit in order to input the modulation signals. However, it is difficult to obtain the two-output type driver, which may have higher properties over the large bandwidth and low price, and the two-output type driver is much more expensive than a single output type driver. Generally, there are problems in that elements such as a phase shifter and the like are needed to obtain optimal modulation, and configuration or adjustment of the circuit becomes complicated.
An overview of a modulator which domain-inverts a part of a substrate is shown in FIGS. 3A and 3B. An optical modulation element 40 domain-inverts a part of the Z-cut LN substrate 21 and a domain-inverted region is denoted by a reference numeral 48. The MZ type waveguides 5 to 8 are formed on the LN substrate 21. Additionally, a modulation electrode 42, ground electrodes 41, 43, 44, and a bias electrode 47 are formed on the LN substrate 21 in which the waveguides are formed.
FIG. 3B is a cross-sectional diagram of the optical modulation element 40 taken along one dot chain line A in FIG. 3A.
In the modulation electrode 42 of the optical modulating element 40, one end of the modulation electrode is electrically connected to the modulation signal input unit 15, and the other end is connected to RF terminals 45, 46. Additionally, the bias electrode 47 is electrically connected to the bias controlling DC input unit 17. The optical modulation element 40 is hermetically sealed in a metal case 18.
A modulation signal and a bias controlling DC signal for driving the optical modulation element 40 are inputted from external of the metal case 18 through a modulation signal input unit 15 and the bias controlling DC input unit 17.
As shown in FIG. 3, a zero chirp modulator can be configured by using the Z-cut LN substrate and by inverting a polarizing direction of the crystal in a part of the LN substrate. Generally, in order to match the impedance of inside the modulator with that of outside the modulator, the line impedance of the interaction portion after branching is needed to be high impedance which is double of the input impedance. For example, when the input impedance is 50 Ω, each line impedance is 100 Ω. Especially, in coplanar waveguide (CPW) which is widely used in an LN modulator, there are problems, in that it is difficult to simultaneously satisfy conditions such as high impedance, speed matching of the electric signal with the light wave, and high modulation efficiency, and so on due to the dielectric constant of the LN substrate.
Accordingly, it is an object of the present invention to provide an optical modulation element module which can solve the above-described problems and is capable of the optical modulation for analog transmission with a low driving voltage and low chirp.